Commuters passing through Grand Central Terminal in New York. Health officials have urged Americans who are vulnerable to limit travel and avoid large crowds

Betsy McKay

As the new coronavirus spreads around the world, health officials are repeatedly assuring the public: Your risk of getting seriously ill or dying is very low—unless you are older or have an underlying condition.

That is a lot of people, including millions of Americans.

Data from China, where the epidemic began, show death rates that are startlingly higher than the average for people age 60 and over, as well as for people with high blood pressure, diabetes, asthma and other chronic conditions. In one large study by the Chinese Center for Disease Control and Prevention, 14.8% of people 80 and older and 8% of people 70 to 79 died. For people with cardiovascular disease, the mortality rate was 10.5%, and 7.3% for those with diabetes. Those rates compared with a 2.3% mortality rate in the overall population of 44,672 patients studied through Feb. 11, China’s CDC says.

Who’s Vulnerable?

Among a large, early group of patients—those diagnosed with the new coronavirus in mainland China through Feb. 11—the death rate was highest for people over 60 and those with underlying illnesses.

Death rate among diagnosed patients...

...by age group

...and by underlying condition

Overall death rate, 2.3%

Cardiovascular disease

Under 10

10.5%

0.0%

Diabetes

10–19

7.3

0.2

Chronic respiratory disease

20–29

6.3

0.2

Hypertension

30–39

6.0

0.2

Cancer

40–49

5.6

0.4

None

50–59

0.9

1.3

60–69

3.6

70–79

8.0

80+

14.8

Death rate among diagnosed patients...

...by age group

...and by underlying condition

Overall death rate, 2.3%

Cardiovascular disease

Under 10

10.5%

0.0%

Diabetes

10–19

7.3

0.2

Chronic respiratory disease

20–29

6.3

0.2

Hypertension

30–39

6.0

0.2

Cancer

40–49

5.6

0.4

None

50–59

0.9

1.3

60–69

3.6

70–79

8.0

80+

14.8

Death rate among diagnosed patients...

...by age group

...and by underlying condition

Overall death rate, 2.3%

Cardiovascular disease

Under 10

10.5%

0.0%

Diabetes

10–19

7.3

0.2

Chronic respiratory disease

20–29

6.3

0.2

Hypertension

30–39

6.0

0.2

Cancer

40–49

5.6

0.4

None

50–59

0.9

1.3

60–69

3.6

70–79

8.0

80+

14.8

Death rate among diagnosed patients...

...by age group

Overall death rate, 2.3%

Under 10

0.0%

10–19

0.2

20–29

0.2

30–39

0.2

40–49

0.4

50–59

1.3

60–69

3.6

70–79

8.0

80+

14.8

...and by underlying condition

Cardiovascular disease

10.5%

Diabetes

7.3

Chronic respiratory disease

6.3

Hypertension

6.0

Cancer

5.6

None

0.9

Source: Chinese Center for Disease Control and Prevention

Those categories potentially apply to a vast and growing swath of the U.S. population. The number of Americans 65 and over rose 27% between 2011 and 2018 to 52.4 million, according to the U.S. Census Bureau, while the under-65 cohort grew 2%. Older Americans make up 16% of the U.S. population.

About 46% of American adults have high blood pressure, according to guidelines by the American College of Cardiology and the American Heart Association. About 34.2 million people, or 10.5% of the U.S. population, have diabetes, according to the U.S. Centers for Disease Control and Prevention.

China’s statistics are from the early weeks of an explosive epidemic in which hospitals were overwhelmed and doctors struggled to figure out how best to care for patients struck with a new disease. Death rates in less-hard-hit parts of China have been lower, and have come down for the whole country over time, according to the World Health Organization.

Precise death rates are also unclear because it isn’t known how many people have actually been infected. Many have either not been ill enough to suspect they had the disease, or haven’t had access to a test.

Still, the data show that people with aging or taxed immune systems have a far more difficult time fighting off Covid-19, the respiratory disease caused by the new coronavirus.

“It’s very clear that this virus has greater mortality and morbidity in older individuals and those with chronic conditions,” says Mark Mulligan, director of NYU Langone Health’s division of infectious diseases and immunology in New York. Morbidity is the rate of a disease’s presence in a population.

Older people are generally at higher risk of becoming severely ill from infectious diseases, such as the flu, Dr. Mulligan notes. As people age, the immune system undergoes “immunosenescence,” gradually losing its ability to mount a response to an infection as robustly as it once did. Precisely how that happens is an active subject of study. Among the changes, the thymus, an organ of the immune system, shrinks starting at young adulthood. T-cells, soldiers of the immune system which mature in the thymus, lose some of their function.

Early Signs

Initial deaths in the U.S. and South Korea show similar demographic patterns.

Deaths by age group, as of March 11

U.S.

South Korea

Under 10

Under 10

10-19

10-19

20-29

20-29

30-39

30-39

40-49

40-49

50-59

50-59

60-69

60-69

70-79

70-79

80+

80+

Unknown

Deaths by age group, as of March 11

U.S.

South Korea

Under 10

Under 10

10-19

10-19

20-29

20-29

30-39

30-39

40-49

40-49

50-59

50-59

60-69

60-69

70-79

70-79

80+

80+

Unknown

Deaths by age group, as of March 11

U.S.

South Korea

Under 10

Under 10

10-19

10-19

20-29

20-29

30-39

30-39

40-49

40-49

50-59

50-59

60-69

60-69

70-79

70-79

80+

80+

Unknown

Initial deaths in the U.S. and South Korea show similar demographic patterns.

Older people often don’t develop high fevers as a result—and a temperature not regularly recognized as a serious fever may be missed or mistaken for another problem, says XinQi Dong, director of the Institute for Health, Health Care Policy and Aging Research at Rutgers University. Thus, says Dr. Dong, “100.5 may be a true fever” possibly indicating the presence of Covid-19.

Lung function also declines with age, making respiratory diseases a particularly serious threat for older people, he says.

Older people are more likely to have serious underlying conditions that also put them—and younger people—at risk of complications from Covid-19. Cardiovascular disease, diabetes and lung diseases all tax the body, making infections harder to fight off, says Bruce Ribner, director of Emory University School of Medicine’s Serious Communicable Diseases Unit. “The system is just not able to put up with that much of an assault,” he says.

High blood glucose impairs the ability of white blood cells to fight infections in people with diabetes, Dr. Ribner says. Viruses can settle more easily into people whose lungs are impaired from a lung disease, he says.

Scientists say they are trying to learn more about the apparent link seen in China between severe Covid-19 and high blood pressure. People with cardiovascular disease often have lung diseases, too, and generally are more susceptible to flu and viral pneumonia, experts say.

Covid-19 has already exacted a heavy toll on older, sicker Americans, claiming at least 22 lives in one nursing home in a Seattle-area outbreak that public health officials are struggling to contain. Health officials say Italy’s aging population is a big reason behind the large outbreak and high current death rate of 5% in that country. Death rates for some older people have also been higher than they are for the overall population in a large outbreak in South Korea.

Italy and China have higher smoking rates, which may be a factor in their high death rates among older adults, says Marc Lipsitch, professor of epidemiology at the Harvard T.H. Chan School of Public Health and director of the Center for Communicable Disease Dynamics. Dr. Lipsitch says he expects mortality rates to be higher for vulnerable people than for the overall population in the U.S., though how much higher is hard to tell.

U.S. health officials have urged Americans who are vulnerable to limit travel and avoid large crowds. The risks to such people are “considerable,” says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“If you are a person with an underlying condition and you are particularly an elderly person with an underlying condition, you need to think twice about getting on a plane on a long trip,” Dr. Fauci said on NBC’s “Meet the Press” this week. “And not only think twice, just don’t get on a cruise ship.”

The U.S. government has urged nursing homes to restrict visitor access, to reduce the risk of spread.

People at higher risk should heed the same precautions that exist for everyone—wash hands regularly, stay away from sick people and keep up-to-date on vaccines for flu and pneumonia, the U.S. CDC and infectious-disease experts say.

They should avoid large gatherings, limit close contact with others when out in public, and stay home as much as possible to reduce risk of exposure, according to the U.S.

That likely means no church or theater. “For now, do not congregate in places with a lot of people,” Dr. Dong says. “I would not go to bingo parlors.”

But a bridge game with a few trusted friends—who aren’t sick—is OK, he says. It’s important to stay socially connected even if physical connection is limited. Connecting by phone or video with older relatives who are alone is a good idea, he says.

Social isolation has been linked to adverse health outcomes, Dr. Dong says. “Being isolated is one of the worst things one can experience.”

Ms. McKay is a reporter for The Wall Street Journal in New York. She can be reached at betsy.mckay@wsj.com.